1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus, a printer or a facsimile apparatus for image formation with hyperchromic toner and hypochromic toner.
2. Related Background Art
As an image forming apparatus for forming a color image, there is already commercialized an image forming apparatus capable of transferring color images by precisely superimposing succession toner images of respective colors, formed on a photosensitive drum serving as an image bearing member, onto a transfer material such as paper supported for example on a transfer drum (transfer film), thereby forming a color image.
In such image forming apparatus, an electrostatic latent image, formed on the photosensitive drum according to an input image signal, is developed with toner of a first color (for example cyan color) to obtain a toner image, which is transferred onto a transfer material such as paper supported on a transfer drum (transfer film). Such transfer process is repeated similarly for other three colors, namely magenta, yellow and black, whereby a color image is obtained by superposed transfers of toner images of four colors on the transfer material.
In the recent electrophotographic image forming apparatus utilizing digital image signals, the latent image is formed by a group of dots of a constant potential on the surface of an image bearing member or so-called photosensitive member, and a solid image portion, a halftone image portion and a line image portion are obtained by changing the density of the dots.
In such method, however, toner particles cannot faithfully be deposited on the dot but tend to overflow from the dot, whereby the gradation of the toner image does not correspond to a ratio of the dot densities in a black portion and a white portion of the digital latent image.
Also in case of increasing the resolution by reducing the dot size in order to improve the image quality, the latent image constituted of finer dots becomes more difficult to reproduce thereby leading to an image lacking sharpness and poor in the resolution and the gradation particularly in the highlight portion. Also an irregular arrangement in the dot is observed as a granularity and deteriorates the image quality particularly in the highlight portion.
Such irregularity is not present in the ink jet recording or in the lithographic printing, and is an unpredictable factor in the image quality and causes a macroscopic low-frequency noise generated by a random distribution of small toner particles of a size of 5 to 10 μm along the dot contour.
A magnified observation of an electrophotographic image reveals that a dot formed by an electrophotographic process does not have a smooth contour as in ink jet recording but is formed by a random distribution of the small toner particles of a size of 5 to 10 μm along the dot contour. Also such dots are not uniformly formed but are uneven, with low density ones and high density ones, also with those of smaller and larger diameters and with non-circular shapes. These factors show almost random fluctuation and include considerable low-frequency components, which lead, as a result, to a visible noise.
Such noise is rendered conspicuous particularly by a difference in the density of the toner and that of the paper. Particularly in comparison with the ink jet recording, there results a significant influence of an optical dot gain, resulting from a distribution of a large number of small toner particles.
These phenomena are principally generated by a fact that small toner particles are used for the dot formation in the electrophotographic process. Also there are various subsidiary factors such as an unsharpening of dot data in the electrophotographic process involving steps of latent image formation, image development and image transfer, an irregular toner scattering resulting from physical properties (electrical resistance, surface roughness) of the copying paper, and a phenomenon resulting from an adhesion force in the development process to be explained in the following.
There is a strong adhesion force (principally a mirror force of toner to a developer bearing member) between the toner and the developing sleeve in case of a single-component developer or between the toner and the carrier in case of a two-component developer, while the toner particles have uneven distribution of charge. Therefore, in peeling off such toner particles with a developing bias voltage to cause a flight toward the photosensitive drum, image formation becomes uneven as the toner particles in a part can easily fly while those in another part do not fly so easily, whereby formation of the dots becomes uneven.
On the other hand, a hyperchromic-hypochromic ink process in the ink jet recording as disclosed in Japanese Patent Application Laid-Open No. 58-39468 is free from the above-mentioned drawbacks the electrophotographic process because the ink jet system is simpler and the high image quality is supported by current paper exclusive for ink jet recording.
Based on an effect of improving the granularity by the hyperchromic and hypochromic inks employed for example in the ink jet recording, it is found that the use of a hypochromic toner in the electrophotographic process is far effective than in the ink jet recording in reducing the visible low-frequency noise, resulting from “a fluctuation in the toner density constituting the dot”, “a fluctuation in the dot area”, and “a fluctuation in the dot shape”.
It is also found that the introduction of the hypochromic toner in the electrophotographic process brings about a revolutionary progress in reducing the optical dot grain which is not a problem in the ink jet recording but has been a serious problem in attaining a high image quality in the electrophotographic process based on a multitude of small toner particles.
For avoiding these drawbacks, there is already proposed a method of employing a pale-colored toner (hypochromic toner) in a highlight area and a dense-colored toner (hyperchromic toner) in a solid image area. Japanese Patent Applications Laid-Open Nos. H11-84764 and 2000-305339 refer to an image forming method for forming an image by combining plural toners of different densities. Also Japanese Patent Application Laid-Open No. 2000-347476 refers to an image forming apparatus employing a combination of a hyperchromic toner and a hypochromic toner of a maximum reflective density less than a half of the maximum reflective density of the hyperchromic toner. Also Japanese Patent Application Laid-Open No. 2000-231279 refers to an image forming apparatus employing a combination of a hyperchromic toner having an image density of 1.0 or higher at a toner amount of 0.5 mg/cm2 on a transfer material and a hypochromic toner having an image density less than 1.0. Also Japanese Patent Application Laid-Open No. 2001-290319 refers to an image forming apparatus employing a hyperchromic toner and a hypochromic toner having an inclination ration of the recording density within a range from 0.2 to 0.5.
However, such prior technologies as explained above have been associated with following drawbacks.
Investigation of the present inventors has revealed that, in such technologies, the gradation and the granularity are improved in a low-density area constituted solely of the hypochromic toner, but the granularity becomes more evident in a medium-density area where the hyperchromic toner and the hypochromic toner are mixedly present.
This is caused by a fact that a state in which the hyperchromic toner is present in a very small amount in the hypochromic toner is extremely unstable in the process condition but is very sensitive visually.
Such instability, which has been avoided in the prior ink jet printer employing six-colored inks (hyperchromic and hypochromic inks) by delicately controlling the ink discharge amount, is in fact the reason why such hyperchromic-hypochromic system has not been adopted in the electrophotographic apparatus.